The present disclosure relates generally to wireless identification systems and methods, and more specifically, but not exclusively, to a radio frequency identification (RFID) system that employs alternate control of non-parallel antennas to make tag identification orientation less sensitive.
Electromagnetic tag readers have been developed in the art to electronically sense the identification of an electromagnetically coupled tag over varying distances. RFID transponders are examples of such tags, which are operated in conjunction with RFID readers (or “interrogators”) for a variety of purposes, to include inventory control and data collection. An item having a tag associated with it is brought into a read zone established by the reader. The RFID reader generates a modulated electromagnetic signal at a carrier frequency. The modulated signal, which carries information, communicates the information at a rate that is lower than the carrier frequency. The RFID reader transmits an interrogating RF signal, which is re-modulated by a receiving tag in order to impart information stored within the tag to the signal. The receiving tag then transmits the re-modulated answering RF signal to the reader, which is often, but not always, mobile or portable.
In passive (and in some active) RFID transponders (tags), antennas connected to the tag's front-end need to produce an output voltage that is above some threshold voltage to power the RFID circuit of the tag. This output voltage is obtained within the tag's antenna, together with the tag's front-end circuitry, via electromagnetic induction with the tag reader's transmitted electromagnetic signal. When sufficient current is induced in the tag, then the output voltage is large enough to operate the RFID circuit, allowing the re-modulation and transmission of the identification signal. In contrast, when the voltage and/or power requirements of the RFID circuit are not fulfilled, the RFID circuit will not resonate. If the received signal strength is not optimal, the distance between the tag reader and the tag must be reduced for continued operation of the tag, thus decreasing the utility of the reader.
In space free of any obstructions or absorption mechanisms the strength of the electromagnetic field is reduced in inverse proportion to the square of the distance. For a wave propagating through a region in which reflections can arise from the ground and from obstacles, the reduction in strength can vary quite considerably, in some cases as an inverse fourth power of the distance. Thus, the distance between a tag reader and a tag and the environment in which a tag is interrogated may both have a significant effect on the success of receiving a response from the tag.
In RFID readers generally, the relative orientation or polarization between the reader and the tag has a strong influence on the strength of the re-modulated radio signal that carries the tag identification from the tag. The more parallel the two are in orientation, the stronger the re-modulated radio signal. As a consequence, circular polarization is desired to provide favorable relative orientation when the tag passes through the read zone, regardless of the tag's orientation. However, these types of RFID readers have had to sacrifice approximately half their power output to do so, thus the advantage is often questionable. The power loss is due to having to provide power to two orthogonal (or perpendicular) antennas simultaneously, for instance to provide circular polarization, thus requiring to half the power to each antenna that would otherwise power a single antenna.